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                [Sdoi2016]探险路线                </h1>
                <p>时间限制：10s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：512MB</p>			</div>
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                                <h3>题目描述</h3><p><div>你所面对的丛林，可以被刻画为n行m列的格点图，其中第i行第j列的格子代表了一块区域，每个格子有整数权值v(</div>
<div>i,j)（可能为负），表明了访问这一块区域的收益或代价。每一个格子最多只能被访问一次，且不可走出地图的边</div>
<div>界，你被要求从第一行第一列出发，到第n行第m列结束，你的目标是最大化途经的所有格子的权值和。因为一些缘</div>
<div>故，你的探险路线受到了一些限制。起初你在起点，之后每一天的行动中，首先你需要选择上下左右中的某一个方</div>
<div>向，沿着这个方向走0步（也就是不走）或任意步；之后重新选择一个方向（可以与原来方向相同，也可以是不同</div>
<div>的方向），沿着这个方向一直走下去，走到地图的某个边界位置结束这一天的探险。探险可以有任意多天，每一天</div>
<div>探险结束的边界位置就是第二天的起点位置，除非这一天就是探险的结束。注意，因为每一块格子只能被访问一次</div>
<div>，且你最终的结束点必须是第n行第m列的位置，所以你需要谨慎计划每一天的路线。你希望知道最优方案下，整个</div>
<div>探险之旅的收益有多大，即你可以获得的权值和最大是多少。</div>
<p></p></p><hr/><h3>输入格式</h3><p><div>第一行输入两个整数，分别表示总行数n与总列数m。之后n行，每行有m个整数，保证绝对值在100000以内。其中第</div>
<div>i行第j列的整数对应了访问第i行第j列区域的收益或代价。3&lt;=n&lt;=800且3&lt;=m&lt;=800</div>
<p></p></p><hr/><h3>输出格式</h3><p><div>输出一个整数，表示最优探险路线中所有被访问格子的权值和。</div>
<p></p></p><hr/><h3>样例输入</h3><pre>10 10
+1 +1 +1 +1 +1 -1 -1 -1 -1 -1
-1 -1 +1 +1 +1 -1 -1 -1 -1 -1
-1 -1 +1 +1 +1 +1 +1 +1 +1 +1
+1 +1 +1 +1 -1 -1 -1 -1 -1 +1
+1 -1 -1 +1 -1 -1 -1 -1 -1 +1
+1 +1 +1 +1 -1 -1 -1 -1 -1 +1
+1 +1 +1 +1 +1 +1 +1 +1 +1 +1
+1 -1 -1 -1 -1 -1 -1 -1 -1 -1
+1 +1 +1 +1 -1 +1 +1 +1 +1 +1
-1 -1 -1 +1 +1 +1 -1 -1 -1 +1</pre><hr/><h3>样例输出</h3><pre>53
</pre><hr/><h3>提示</h3><p>没有写明提示</p><hr/><h3>题目来源</h3><p>By AHdoc命题 鸣谢Loi_DQS上传</p>
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